A number of techniques have been developed for characterizing the surface topography, voltage potential and capacitance distribution of semiconductor devices. These techniques have been developed in response to the advance of semiconductor technology, in which the dimensions of processed semiconductor devices are becoming ever smaller. This diminution in device scale renders both physical and electrical analyses more difficult to perform. Traditional methods of electrical measurement, such as direct mechanical probing, tend to become difficult or impossible to carry out at such reduced scale. Hence, efforts have been directed to devising electrical analysis instruments which are non-destructive, contactless, and which exhibit improved spatial resolution.
Included among recently developed electrical analysis instruments are microscopies based upon local interactions between a probe having a sharpened tip and a sample surface. Such interactions include electron tunneling, atomic force, magnetic force, as well as thermal, optical and electrostatic coupling. Atomic force microscopes, for example, operate by sensing minute deflections of a cantilever to which is attached an atomically sharp tip. In a non-contact mode, attractive forces between the tip and sample induce bending of the tip and cantilever to allow determination of surface potential (potentiometry). Similarly, in scanning tunneling microscopes potentiometry is effected by monitoring the tunneling current between the tip and sample.
Unfortunately, capacitance and potentiometry measurements performed using atomic force microscopes have been limited by the relatively slow mechanical response of the force-sensing cantilever and associated feedback electronics. Although scanning tunneling microscopes may be operated in a contact mode so as not to be limited by the mechanical response of the cantilever, conventional scanning tunneling microscopes have typically been employed to provide a measure of the average potential of high-speed signals present on the surface of a sample. That is, it has been required to use other techniques to provide the equivalent of an oscilloscope-like trace, i.e, a map of high-speed electrical waveforms present on the surface of a sample. Such high-speed potentiometry has been demonstrated using various microwave probes, but these have required contact pads on the order of 10 microns. Hence, probing techniques have been generally not be utilized in applications involving devices having dimensions on the order of only a few microns.
High-speed potentiometry has also been performed using optical techniques, in which a laser beam is modulated by a high-speed electrical waveform proximate the surface of a sample. However, the lateral resolution of optical potentiometry systems are limited by the diameter of the illuminating laser beams, which are typically on the order of two to three microns. Although high-speed potentiometry using electron beams is capable of yielding high resolution, space-charge effects are expected to place an upper limit on the speed of waveforms which may be analyzed using this technique. Moreover, electron-beam potentiometry requires that the sample be placed in a vacuum environment.